In the process for forming high contrast images by development of silver halide photographic elements necessary to obtain useful images for graphic arts processes, special developers known in the art as "lith" developers are used. The high contrast is achieved by means of the infectious development as described in Journal of the Franklin Institute, vol. 239, 221-230 (1945). These developers exhibit an induction period prior to the development of exposed silver halides, after which the infectious development occurs, which gives rise to the high contrast.
The typical "lith" developer contains only a single developing agent of the dihydroxybenzene type, such as hydroquinone. In order to enhance the infectious development, "lith" developers contain a low content of alkali sulfite. This low sulfite content renders the developer more prone to aerial oxidation, especially when it is used in combination with processing machines and, more particularly, with Rapid Access type processing machines, where the developer degradation is accelerated.
Moreover, the delay in start of development caused by the long induction period of hydroquinone developers lengthens the processing time and delays access to the finished material. While the induction period has been eliminated and processing time reduced by using the so called "Rapid Access" developers, which contain both hydroquinone and a superadditive developing agent such as phenidone or metol, these Rapid Access developers are not useful for lithographic purposes because they cannot produce the necessary high contrast. This is because Rapid Access developers have a high sulfite content which prevents infectious development and causes a lower contrast than "lith" developers.
Processes for obtaining a high contrast development of silver halide photographic emulsions have been disclosed. They are based on the addition of a hydrazine compound to a negative surface latent image type silver halide emulsion and on the development of the emulsion at a pH of about 12.8. The use of hydrazine compounds allows the use of auxiliary developing agents in combination with the dihydroxybenzene developing agents in order to increase the developing capacity. It also allows the use of relatively high sulfite concentrations to protect the developing agents against oxidation, and thereby increasing the developer stability. Anyhow, the high pH level necessary to obtain the high contrast from the use of hydrazine compounds makes the life of the developing solution relatively short.
The process which makes use of hydrazine is disclosed in U.S. Pat. Nos. 2,419,975; 4,168,977 and 4,224,401. Modifications and improvements to the hydrazine process are disclosed in U.S. Pat. Nos. 2,419,974; 2,410,690; 4,166,742; 4,221,857; 4,237,214; 4,241,164; 4,311,871; 4,243,739 and 4,272,614.
Despite the improvements which have been made in the hydrazine process, a remaining inconvenience was the relatively low stability of the developer to aerial oxidation, which is a consequence of the high pH required to achieve the desired high contrast.
Contrast promoting agents have been described in U.S. Pat. No. 4,269,929 and in European Patent Application Ser. No. 155,690 which, incorporated in the developing solution, allow the photographic element, including the hydrazine compound, to reach the desired high contrast at a low pH. It would be still desirable to obtain a photographic element providing a high contrast upon development in the presence of a hydrazine compound with a conventional Rapid Access type developer solution, without the necessary addition to said solution of ingredients of uncommon use such as the above mentioned contrast promoting agents.